In the production of silicon crystals grown by the continuous Czochralski (CCZ) method, polycrystalline silicon is first melted within a crucible, such as a quartz crucible, of a crystal pulling device to form a silicon melt. The puller then lowers a seed crystal into the melt and slowly raises the seed crystal out of the melt. As the seed crystal is grown from the melt, solid polysilicon or liquid silicon is added to the melt to replenish the silicon that is incorporated into the growing crystal.
Desired amounts of dopants are added to the melt to modify the base resistivity of the resulting monocrystalline ingot. In some instances, it is desirable to use volatile dopants in the silicon crystal growth process, such an indium, antimony, and gallium. For example, it is desirable to use indium as a dopant in crystals used for solar structures due to an increased performance in indium-doped solar structures as compared to boron doped solar structures. Use of volatile dopants in the CCZ process presents several challenges, however. For example, due to the volatile nature of such dopants, a significant amount of dopant may be lost to evaporation during the process, making the crystal growing process costly. Additionally, loss of dopant during the growth process makes controlling the dopant concentration of the melt difficult.
While some known systems address some of the above problems associated with the use of volatile dopants, most known systems for doping a melt in a CCZ process with a volatile dopant do not provide a sufficiently uniform resistivity profile at the seed end of grown crystals. For example, to reduce losses associated with evaporation of volatile dopants, some systems add dopants to the melt just prior to initiation of the crystal growing process. Such systems generally add dopant to an outer melt zone to avoid disturbing the melt surface in the inner melt zone, which can result in a loss of crystal structure during the growth process. Diffusion of the dopant to the inner melt zone is relatively slow. Thus, the dopant is primarily transported to the inner melt zone by the physical flow of the liquid melt towards the inner melt zone. Because such systems generally add dopant to the outer melt zone, the initial portion of the grown crystal (i.e., the seed end) has a significantly lower dopant concentration than the remainder of the crystal, and thus, a higher resistivity. This region of the grown crystal is sometimes referred to as the “high resistivity transient region”. The high resistivity transient region is not typically used in subsequent device fabrication, resulting in productivity losses and increased costs of production.
Accordingly, a need exists for an apparatus and method that reduces or eliminates the high resistivity transient region in semiconductor or solar grade crystals grown according to the CCZ method.
This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.